Insulated glass units (IGUs) generally comprise a pair of glass sheets, maintained in a spaced-apart relationship to each other by a spacer assembly, and a sealing assembly which extends around the periphery of the inner facing surfaces of the glass sheets to define a sealed and insulating air space between the glass sheets. Typically, the spacer assembly is a hollow form which extends around the periphery of the inside facing surfaces of the glass sheets and which is filled with a water-absorbent material, such as a molecular sieve or another dehydration element, to keep the enclosed hollow space dry. The inner surfaces of the glass sheets are attached to the outer surface of the spacer assembly by means of a sealant or adhesive. Generally, the sealant or adhesive is also used to seal the edges of the insulated glass unit so as to establish a barrier which prevents moisture from penetrating into the interior annular space of the unit.
The sealant must have a combination of properties for satisfactory use. For example, the sealant must have a very low moisture vapor transmission rate (MVTR) so that moisture is prevented from entering the dry annular space between the panes of glass. Moisture in such space tends to condense on the interior faces of the panes, creating visibility and aesthetic problems. If the sealant does not have a satisfactory MVTR, the longevity of the insulated unit may be severely reduced. The sealant should have good elongation and flexibility so that it “yields” during contraction and expansion of the insulated glass structure, for example, to relieve stress on the glass caused by changes in temperature. The sealant desirably also forms an excellent bond with the glass which is not degraded over long periods of use when exposed to sunlight, moisture, and large temperature changes. Tensile adhesion strength is an important indicator of bond strength.
Two of the major types of sealants currently used in the insulated glass industry are: (A) thermoplastic one-part hot melt butyl type sealants, and (B) the chemically-curing thermoset sealant products generally from the generic families of polysulfides, polyurethanes, and silicones. Hot melt butyl insulated glass sealants have been used with moderate success for a number of years in the insulated glass industry. However, there are significant shortcomings with this technology that have limited the application of hot melt butyl insulated glass sealants. Primarily, the hot melt butyl is a thermoplastic material, and not a thermoset material. Thermoplastic sealants are well known to soften when exposed to heat. Therefore, the insulated glass units sold in the marketplace which employ thermoplastic sealants are known to flow or deform, when placed under load, to relieve such stresses. This characteristic is exaggerated at high temperatures, which can occur in insulated glass units, especially those utilizing solar control glass. As a result, insulated glass units made with hot melt butyl sealants have difficulty passing stress and temperature tests common in industry, and are often limited for use in relatively small, light insulated glass units. Additionally, extreme care must be taken to support the insulated glass unit during handling, shipping and installation, resulting in additional costs. Furthermore, the hot melt sealants previously employed must be applied to the insulated glass units at temperatures exceeding 300° F. These high temperature requirements often present increased manufacturing costs, for example due to higher energy consumption and the need for specialized high-temperature equipment, as well as operational and safety challenges. Attempts to utilize lower temperature hot melts have been known to cause flow problems with the sealant.
More recently, sealants based on polyurethane chemistry have been used for insulated glass units. These polyurethane-based sealants employ polymeric polyols, which are reacted with polyisocyanate to form a sealant. Various types of different polyols have been proposed for use in such sealants. Hydroxyl terminated polyols with very non-polar backbones (e.g., hydroxyl-terminated polybutadiene) can be used to introduce hydrophobicity into polyurethanes. However, polyols having a polybutadiene backbone, for example, usually have a much higher viscosity than those based on a polyether backbone. To reduce the viscosity of hydroxyl-terminated polybutadienes, one can either blend polyether polyols into the polyol mixture or make prepolymers with increased —NCO percentage. These approaches in general are not ideal because the final polyurethane products tend to have inferior hydrophobicity.
Thus, there is a need for improved curable compositions having relatively low viscosity for easier application that, once cured to form polyurethane-based sealants, are sufficiently hydrophobic for moisture-sensitive applications (such as their use as insulating glass sealants) and that have improved mechanical properties.